U.S. patent number 6,064,913 [Application Number 09/334,823] was granted by the patent office on 2000-05-16 for multiple pulse stimulation.
This patent grant is currently assigned to The University of Melbourne. Invention is credited to Graeme Clark, Laurence Irlicht.
United States Patent |
6,064,913 |
Irlicht , et al. |
May 16, 2000 |
Multiple pulse stimulation
Abstract
A stimulation strategy for cochlear implants seeks to
approximate the time domain response of a patient's neural system
to electrical stimuli, to the time domain response of a normal
hearing person to a corresponding acoustic stimulus. The strategy
is designed to induce in the neurons of a patient a time domain
response to an acoustic signal which is similar to, or approximates
the time domain response induced by the normal processes in a
healthy person. Various implementations are disclosed.
Inventors: |
Irlicht; Laurence (Brighton,
AU), Clark; Graeme (Eltham, AU) |
Assignee: |
The University of Melbourne
(Parkville, AU)
|
Family
ID: |
25223186 |
Appl.
No.: |
09/334,823 |
Filed: |
June 17, 1999 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
|
817481 |
Apr 16, 1997 |
5991663 |
|
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Current U.S.
Class: |
607/57 |
Current CPC
Class: |
A61N
1/36038 (20170801) |
Current International
Class: |
A61N
1/36 (20060101); A61N 001/36 () |
Field of
Search: |
;600/554 ;607/55-57 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Kamm; William E.
Attorney, Agent or Firm: Gottlieb Rackman & Reisman
PC
Parent Case Text
This application is a division of application Ser. No. 08/817,481
filed Apr. 16, 1997 and now U.S. Pat. No. 5,991,663.
Claims
We claim:
1. A method for producing sets of stimuli to be applied by a
stimulus generator to selected electrodes in an auditory prosthesis
in response to an electrical signal corresponding to an acoustic
signal, said method comprising:
processing said electrical signals in accordance with a
predetermined instruction set, said instruction set performing the
steps of:
analyzing the electrical signal to determine the electrodes to be
stimulated;
for each electrode to be stimulated, determining a set of stimuli
so that the neural structures of a patient responsive to each
electrode in response to said set of stimuli have a time domain
response which is an approximation to the time domain response of a
normal hearing person to said acoustic signal, said stimulus set
including a plurality of stimuli, and each stimulus including at
least the amplitude of the stimulus, and the timing of the
stimulus; and
providing control signals to said stimulation generator to cause
said stimulation generator to produce said sets of stimuli.
2. A method according to claim 1, wherein said step of analyzing
further comprises determining which of a predefined group of
acoustic features of a portion of the electrical signal corresponds
to features of sets of stimuli, wherein the stimuli required to be
presented for a given electrical signal are determined by
successively analyzing said electrical signal, and obtaining the
appropriate control signals corresponding to sets of stimuli.
3. A method according to claim 1, further comprising analyzing said
electrical signal within a plurality of frequency channels, and for
each channel performing a separate analysis corresponding to
approximating the time domain response of part of the neural
structures corresponding to that population of the nerve fibers
most responsive to the frequency channel.
4. A method according to claim 3, wherein said determining step is
performed using a model of neural response to determine an
approximation of the time domain response of a normal hearing
person to a sample of said acoustic signal, said approximation
being used to derive a desired stimulus amplitude.
5. A method according to claim 4, further comprising determining
said amplitude by calculating a desired stimulus function for the
patient corresponding to the approximation of the time domain
response of a normal hearing person, and deriving the desired
stimulus amplitude.
6. A method according to claim 4, further comprising relating said
desired stimulus function to said desired stimulus amplitude using
a look-up table.
7. A method according to claim 1, wherein each electrode is
associated with neurons having a relative refractory period further
comprising generating for each stimulated electrode, a set of
stimuli including multiple pulses presented at a rate much faster
than said relative refractory period of the associated neurons.
8. A method according to claim 7, further comprising generating the
stimulus set for each electrode such that the adjacent neurons
exhibit a population per-stimulus time histogram which is an
approximation to the population per-stimulus time histogram
generated by a corresponding acoustic stimulus in a normal hearing
person.
9. A method according to claim 8, further comprising using said
instruction set to determine the sets of stimuli for each
stimulating electrode in accordance with a technique selected from
the set comprising a model of neural response, predetermined
patient response data, and telemetry from said stimulating
means.
10. A method according to claim 7, further comprising providing at
least some of the stimulus sets selected to excite different
population bands of neurons.
11. A method according to claim 10, further comprising using a size
of the population bands selected in accordance with experimentally
derived patient data in order to maximize the perception of sound
by patients.
12. A method according to claim 11, further comprising using
electrode bands customized for each patient.
Description
TECHNICAL FIELD
The present invention relates to methods and devices for providing
electrical stimuli, and a strategy for electrical stimulation, for
auditory prostheses such as cochlear implants.
BACKGROUND ART
Cochlear implants of various types have been proposed and
constructed. For the purposes of explanation of the present
invention, reference will be made to arrangements such as are
commercially available from Cochlear Limited. However, it will be
appreciated that the present invention is equally applicable to
other types of auditory prostheses. An intracochlear electrode
array is surgically implanted in a patient, together with a
receiver stimulator unit for providing electrical stimuli to
selected electrode pairs within the array. The receiver stimulator
unit is connected, via an inductive transcutaneous link or a direct
percutaneous connection, to an external sound processing device and
microphone.
The present invention is concerned principally with the process of
selecting appropriate stimuli, and with the actual stimulus pulses
delivered in response to the acoustic stimuli. According to known
arrangements, stimuli may be applied between different pairs of
electrodes, to provide different modes of stimulation. In general,
the electrode pair selected is related to the pitch of a detected
tone. In this case, stimuli have generally used a consistent pulse
timing and shape, and amplitude is determined by reference to the
amplitude of the detected sound signal. It is also known to
stimulate at a rate related to a detected tone, so as to induce a
pitch percept.
However, it has been determined that the perceptions of patients in
response to these stimuli are different from the perceptions using
the normal hearing mechanisms. It has been determined that, in
particular, the response of the auditory nerve to such stimulation
is quite dissimilar to the neural response of a normally hearing
person to the same sound.
In a paper by Parkins et al, entitled "A fibre sum modulation code
for a cochlear prosthesis", Annals of the New York Academy of
Sciences, 1983 at p 490, the authors discuss providing stimuli in
such a way as to mimic the neural response of a normal hearing
person to acoustic stimuli. The stimulus waveform is modified,
using a complex mathematical model, so that the post stimulus time
histogram approximates that of the normal hearing case. However,
the arrangement described is not suited for real time processing so
as to facilitate implementation in an implantable or portable
device.
U.S. Pat. No. 4,495,384 to Scott et al discloses a real time
processing arrangement for a cochlear implant. This disclosure does
not describe a system which accounts for the refractory period of
nerve fibres, and as a result the stimuli produced do not provide a
neural response having a time domain waveform similar to the normal
hearing case.
In a paper by Motz and Rattay, (1988), "Signal processing
strategies for electrostimulated ear prostheses based in simulated
nerve response", the authors discuss the problems associated with
hyperpolarisation of the auditory nerve fibres, and consequent loss
of perception of higher formants by patients. The stimuli were
simulated as if presented from a single electrode. The authors
propose the use of further pulses after the initial stimulus pulse,
the later pulses having considerable linear increases in amplitude,
to improve the perception of higher formants. There is no
disclosure of selecting pulses so as to produce a desired
post-stimulus time histogram in the auditory nerve structures.
It is an object of the present invention to provide a practical
arrangement for generating electrical stimuli so that an auditory
nerve response is produced Which better approximates the time
domain response of the neural structures of a normal hearing person
to a given acoustic stimulus.
SUMMARY OF INVENTION
According to one aspect the present invention comprises a cochlear
implant device, comprising processing means for receiving an
electrical signal corresponding to an acoustic signal, and
stimulation means adapted to provide electrical stimuli to the
cochlea of a human, said stimulation means including an electrode
array comprising a plurality of electrodes operatively located
within the cochlea, said device being arranged so as to permit
selected electrodes to be provided with stimuli, said stimulation
means being responsive to control signals received from said
processing means,
characterised in that said processing means processes said
electrical signals in accordance with a predetermined instruction
set, said instruction set determining the stimulation to be applied
in response to the acoustic signal including the electrodes to be
stimulated, the amplitude of the stimuli, and the timing of the
stimuli, said processing means providing control signals to said
stimulation means to cause said stimulation means to produce a set
of stimuli, said set including for at least one electrode a first
stimulus pulse and at least one further pulse within the refractory
period of at least a substantial number of the nerve fibres
stimulated by said first pulse, the set being selected such that
the neural structures of a patient in response to said set have a
time domain response which is an approximation to the time domain
response of a normal hearing person to said acoustic signal.
The instruction set preferably analyses the electrical signal
corresponding to the acoustic signal, so as to identify portions of
the signal as corresponding to one of a group of predefined
features. These may include, for example, phonemes, tones or
chords. A look up table may be provided which provides stimulus
sets which have been determined to operatively induce an
appropriate time domain neural response corresponding to the
acoustic feature. Alternatively, the neural response may be
directly calculated. This stimulation is then presented via the
electrode array.
The sets of stimuli may be selected so as to stimulate selected
populations of nerve fibres at different times, so as to take
account of the refractory period of the nerve fibres. The sets of
stimuli may also take into account the responses of specific nerve
fibres, and are preferably tailored to the responses of a
particular patient. For example, each patient may have different
degrees of array insertion, some electrodes may not be active
post-operatively, and different patients have different degrees of
nerve survival. The stimulus sets are chosen so as to allow the
nerve response to the stimuli to approximate the time domain
response of normal hearing patients to the respective acoustic
stimulus. This response may be determined with reference to the
post stimulus histogram, inter-spike intervals histogram, and/or
the period histogram of individual nerves, or of chosen bands or
populations of nerve, or by other telemetry from the stimuli.
The stimulus sets may be designed in advance, using preferably a
model of neural response and statistical and/or mathematical
analysis. Alternatively, the stimulus sets may be calculated in
real time. The stimulus sets may be calculated to take into account
single unit effects, or population effects, or preferably both. The
present invention allows tailoring of the stimuli, both for
different acoustic inputs, and for the peculiarities of individual
patients. It is believed that the closer the time domain waveform
of the evoked auditory response via electrical stimulation can be
brought to that experienced by a normal hearing person, the easier
it will be for patients to interpret the precepts produced.
BRIEF DESCRIPTION OF DRAWINGS
The invention will be described with reference to the accompanying
drawings, in which:
FIG. 1 illustrates schematically an aspect of the theoretical basis
fro the present invention;
FIG. 2 illustrates in block form processing according to the
present invention;
FIG. 3 illustrates graphically theoretical and measured plots of
spike rate against stimulus function;
FIG. 4 illustrates a sample period histogram of a population of
nerves;
FIG. 5 illustrates a multiple pulse histogram according to one
embodiment of the present invention;
FIG. 6 illustrates a histogram produced according to standard
techniques;
FIG. 7 illustrates a histogram according to an embodiment of the
present invention;
FIG. 8 illustrates current levels required to produce the output of
FIG. 5;
FIG. 9 illustrates the spread of action potentials excited by the
stimulation;
FIGS. 10 and 11 illustrate further implementation of the present
invention in block form; and
FIG. 12 illustrates schematically a cochlear implant system;
FIG. 13 illustrates the time and power signals associated with a
phoneme;
FIG. 14 illustrates firing probabilities for various bands of
neurons;
FIG. 15 illustrates the probability of spikes in various time
periods for a particular selected band; and
FIG. 16 illustrates a sample refractory function.
DESCRIPTION
The present invention relates to a broad principle for applying
electrical stimuli to patients with acoustic prostheses. It is
emphasised that whilst the present invention is described with
reference to a specific implementation, a wide variety of possible
implementations exist. For example, different models for neural
response may be used to estimate the required stimuli, and
different stimulation arrangements, for example percutaneous
connection, may well be used.
A cochlear implant system of the type contemplated is illustrated
in FIG. 12, and comprises in general terms a microphone 1 which
receives sound signals and passes a corresponding signal to the
speech processor 2. The speech processor processes the received
electrical signal to produce a set of stimulus data. This is
transmitted, together with power, from the external coil 3 to
internal coil 4, and then to the receiver stimulator unit (RSU) 5,
which then provides stimulus pulses to the selected electrode pairs
of electrode array 6 so as to stimulate the nerve fibres and
provide a percept of sound to the user.
The neural response cannot be derived as a trivial function of,
say, the input sound signal. One aspect of the difficulty of
accurately simulating the response relates to the complexity of the
system. The normal hearing ear has approximately 30,000 nerve
fibres, each of which can reach action potential independently of
the others at any time during the stimulus. It is not possible to
generate electrical pulses which cause this system to behave
identically to the normal hearing situation.
One aspect of the implementation of the present invention relates
to varying the numbers, amplitude, shape and rate of the pulses to
evoke an approximation of the NHNR. This is achieved in particular
by evoking the correct number of action potentials (counted across
either one representative fibre, or alternatively across the
population of fibres) within each "phase" of the waveform, or
averaged over many appropriately chosen phases of the waveform.
This phase corresponds to the available divisions within the
stimulation period, which is limited by the modulation rate--if the
modulation rate was 4 times the frequency of a tone, then each
"phase" would be one quadrant of the waveform. This results in a
"staircase" approximation to the NHNR which is very similar to that
of the acoustical case. The specially designed stimuli are applied
across each period of the simulated tone, and are designed via
computer simulation, and mathematical analysis, of the auditory
nerve's response to both auditory and electrical stimulation.
Modifications to the waveform are used to improve the
spatio-temporal neural response. These modifications include (but
are not limited to) modifications to the number, amplitude,
spacing, and width of the pulses to better simulate the
spatio-temporal pattern of the NHNR. These degrees of freedom are
provided in conventional systems, but are generally not
exploited.
The present invention allows for various types of temporal response
to be induced, as is required in various applications. One approach
is to utilise the present invention to evoke as near an
approximation as possible to the correct population per stimulus
time histogram, by applying several pulses per stimulus period. The
pulse amplitudes may be chosen so as to generate the correct number
of action potentials in each part of the waveform. The pulse sizes
may be chosen using various means, examples of which are discussed
below.
A further aspect of the present invention is that by utilising the
refractive properties of neurons, it is possible according to the
present invention to provide stimuli such that different bands
within the stimulation range of an electrode (SRE) fire at
different times. This allows for the stimuli to evoke desired
inter-pulse timings within each band, so that the phase
relationship between the bands in normal hearing can be
approximated. The size of the bands chosen may be selected, so that
the band size may be selected to be that size which provides the
best percepts for the patient. This may be customised for each
patient.
The present invention is described largely in the context of
available implant systems, which utilise a standard biphasic pulse.
Altering the pulse shape will necessarily alter the time domain
response of the associated neural structures. The present invention
is not limited in scope to the use of existing or standard pulse
shapes, although clearly alternative pulse shapes will alter the
details of the effects noted above.
In order to more fully understand the present invention, we will
initially briefly consider its theoretical basis. In a conventional
cochlear implant, the neurones of the auditory nerve are stimulated
by application of a series of biphasic currents between electrodes
of the electrode array. Each biphasic stimulation causes a group of
neurones to fire. The number of neurones that fire due to a
stimulation is determined by such factors as relative location of
the group of neurones to the stimulating electrodes, and the
history of stimulation of those neurones. If many of the neurones
are in a refractory period due to past stimulation then the
application of new stimulation will not cause as many of them to
fire as would be the case if they were being stimulated for the
first time.
It is further desirable according to the present invention to
provide an estimate which is representative of the temporal
response of the wider population of nerves, not merely those close
to a single electrode. In order to overcome variations in neuron
response due to location with respect to the stimulating
electrodes, the neurones can be considered as divided into strips,
each of which are assumed to contain neurones that are equally
stimulated by the application of a given stimulus pulse. This is
illustrated schematically in FIG. 1. Neurones 12 in the region of
electrodes 10, 11 are notionally divided into strips, labelled i,
i+1, and so forth.
Suppose that a single biphasic stimulation is applied between
electrodes 10,11, at various amplitudes, and the ith strip of
neurones monitored. Whilst of course in practise any given pulse
will stimulate multiple strips, it is assumed that this strip
contains the neurones most responsive to the stimulating electrode.
The stimulus function S.sub.k describes the neural response from
the kth pulse. A.sub.k is the amplitude of the kth pulse. It is
possible to make up a table for each strip relating A.sub.k to
S.sub.k.
In practice the neurones are not stimulated by isolated biphasic
pulses but by a series of stimulations. Each electrical stimulation
will elicit a neuronal response from a single strip of neurones of
NI.sub.k action potentials, where N is the number of neurons in
that strip, and I.sub.k is the averaged probability of any neuron
from that strip acheiving action potential during pulse k. It is
known that the pulse in a series of stimulation pulses that elicits
I.sub.k has the same amplitude as the isolated pulse that elicits
S.sub.k where S.sub.k and I.sub.k are related by: ##EQU1## Let the
pulse period equal T. (n+c) is the length of the relative
refractory period divided by T, and .gamma.(k-i) is one minus the
refractory function measured for time kT since the last action
potential.
Therefore it is possible to generate a particular I.sub.k by
determining Sk and then looking up the appropriate amplitude of the
biphasic pulse A.sub.k to be applied.
So far it has been shown how to determine the amplitude of the
biphasic stimulation to be applied in order to elicit a desired
neural response I.sub.k in the ith stimulated strip of nervous
tissue.
Linking of Acoustic Signals to Neural Response
Current speech processors used in cochlear implant technology rely
on extracting significant features of speech. For example, using
the SMSP process, electrical signals corresponding to received
sound signals are processed by means of band pass filters, eg. 16,
to provide a signal corresponding to amplitude in each channels. A
selected number of said amplitude signals having the greatest
amplitude, e.g.6, are used to modulate the amplitude of the
stimulation pulse.
In order to incorporate the present invention into such a system it
is necessary to calculate the I.sub.k which would arise in the
normal hearing situation in each band wherestimulation is to occur.
This I.sub.k may be calculated by use of an approximate model of
the cochlea and normal neuronal response. See for example Parkins
et al "A Fibre Sum Modulation Code for a Cochlear Prosthesis",
Annals of the New York Academy of Sciences, 1983 p490, or one of
the many other published models.
The I.sub.k is then mapped to the appropriate S.sub.k by means of
the equation above, and the map of A.sub.k to S.sub.k is used in
order to determine the amplitude of the biphasic pulse to be
applied. This process is described in FIG. 2. An input signal 20 is
processed by software 21 in order to extract a particular feature
or set of features. This process may be a conventional cochlear
implant type, for example SMSP or identification of formants.
Alternatively, it may be a software process to recognise phonemes
or similar features, such as discrete musical tones. The recognised
feature is referenced via look-up table 22 to provide a desired
normal hearing neural response, I.sub.k, which corresponds to a
percept of the feature extracted. S.sub.k can then be determined
with reference to the equation above. An amplitude A.sub.k for each
pulse can then be derived from look-up table 24. Simultaneously,
according to this implementation, the input signal 20 is processed
25 so as to select an electrode pair for tonotopic stimulation. The
stimulus is then determined 26 by combining the derived A.sub.k
with the electrode site selected at 25, to provide a set of stimuli
to electrode array 27.
The look up table may be provided using any conventional memory
device. The first table stores the required type of patient
percept, that is, the feature extracted (e.g. a phoneme, or a
tone), with corresponding normal hearing neural response patterns.
The other input to the table is the required volume level of the
perception. The output of the lookup table is a set of electrical
stimuli which evoke the desired neural response. These are
preferably calculated off-line via methods similar to those
described above, and stored. This arrangement allows for a
reduction in processor capacity, as it is not necessary for whole
waveforms to be fully calculated.
The second lookup table 24 requires as input the width of the
stimulation pulses, the rate of stimulation, and the desired
stimulus function (S.sub.k), and returns the amplitude A.sub.k of
the stimulus required for this. The values for the lookup table may
be obtained in a variety of ways. One approach is to use animal
studies with a variety of pulse rates at a variety of amplitudes
and rates (for each pulse width). From the responses measured, the
`s` function can be calculated.
FIG. 3 lists the expected spike rates for biphasic pulses as a
function of the S function (for given pulse rates and pulse width)
versus the actual experimental results. The experimental results
were obtained by putting in fixed width biphasic pulses at various
pulse rates and intensities, and graphing the neural response
rates. The theoretical values may be calculated as follows.
Assume the stochastic process describing the timing of the action
potentials is a self exciting point process (Snyder and Miller,
1991). Define the number of spikes (events) to time t as Nt ,then
at any time t, the time since the last spike equals t-t.sub.Nt. The
intensity of the point process (Snyder and Miller, 1991) is equal
to s(t)r(t-t.sub.Nt), where s(t).gtoreq.0 is a stimulus related
function, depending on time (determined by the properties of the
neuron and also the signal presented to the neuron) and
r(.).gtoreq.0 is a refractory function, which lowers the rate of
action potential generation as a function of the time since last
action potential. r(.) is determined solely by the properties of
the neuron, and possibly also by the type of stimulus (electric or
acoustic), and is independent of the size of the stimulus.
Consider a system where the s function is a set of identical pulses
spaced at a period of T with the width of each pulse W being less
than the dead time of the neuron. Let the refractory function
r(t-t.sub.Nt) be constant over the following regions: ##EQU2##
Define ##EQU3## A is the probability of there being no points
assuming a Poisson rate of s(t) during the pulse, and no refractory
effects. .alpha..sub.n is the size of the refractive function,
where the last action potential occurred n pulses ago. Define N as
the smallest integer such that (N+1)T-W.gtoreq.b. Then, the steady
state average rate of neural firing equals: ##EQU4## The values for
lookup tables which relate the S function to the electrical
intensity, for given conditions, may then be derived. This may be
done in a number of ways.
A relatively simple method involves simply measuring the `S`
function directly for a given pulse set up by measuring the neural
response under a number of conditions of pulse rate and
intensity.
For instance, from FIG. 3, a pulse at an intensity of about 35 when
presented at 200 pps would equate to an S function of about 10, and
increases in intensity will approximately relate to increases in S
function in a linear plus offset relationship.
Alternatively, at 200 pps, to produce an S function of, say 20, a
stimulus intensity of about 40 is required.
Of course, further research may result in a more detailed
representation of the relationship, but this simple initial
approach provides a reasonable representation.
An alternate method of determination of the necessary current
values for a given pulse rate and width would be to (with each
patient) apply a series of pulses at fixed rate and width, and
determine the threshold and comfortable levels of current. Then, a
second parameter which indicates the size of the effective `S`
function at each level could be determined either by masking
studies, or alternately by experiments where such a parameter is
changed, and the perceptual response noted, ie. a particular sound
could be coded, and then repeatedly played to the patient, under
the assumption of a given proportionality between the current
intensity, and the S function. The proportionality which returns
the `best` response--either in terms of naturalness or in terms of
signal discriminability, could be stored in the look up table.
Thus, in this particular implementation, there would be three
parameters: a threshold and comfortable level current, and a scalar
parameter relating the current intensity (at a given pulse rate) to
size of the `S` function. Note that this would need to be done for
each electrode stimulation combination. (ie. monopolar on each,
bipolar on each pair, etc).
It will be appreciated that the pulse timing may be determined in
various ways, within the scope of the present invention. In a
simple implementation, a constant pulse rate may be used for all
electrodes. This rate must of course be much faster than the
relative refractive period, typically 20 ms, and is preferably less
than 1 ms. A preferred implementation uses a pulse rate for each
electrode such that the rate is an integral multiple of the
characteristic frequency of the adjacent neural population.
FIG. 10 illustrates in block form an alternative implementation of
the present invention. In this case, the received acoustic signal
is processed by a transducer, and then enters a filter bank with n
outputs. Illustratively, this may be 6. For each channel, a model
of neural response for that part of the neural structure is used to
produce a normal hearing neural response (NHNR) for that part of
the acoustic signal falling within the channel. The S.sub.k can
then be calculated using the equation shown above. The S.sub.k can
be related to A.sub.k in a look-up table, as previously discussed.
This A.sub.k can then be used as the basis for an instruction to
the RSU to stimulate the appropriate electrode pair at amplitude
A.sub.k.
FIG. 11 illustrates a related implementation to FIG. 10. The
distinction is that for each channel output from the filter bank,
FFT techniques are used
to derive a fundamental tone. Using a similar process to that
described in relation to FIG. 1, this tone is related to a NHNR via
a look up table, the S.sub.k calculated, and the corresponding
A.sub.k determined from a further look-up table. A stimulus
instruction is then sent to the RSU based on the determined
A.sub.k, and the electrode site corresponding to the tone. This
process may be performed for each channel, or for a selected set of
channels, determined via the SMSP technique, which have the
greatest amplitude.
FIGS. 8 and 9 illustrate the principle of the present invention. To
cause a population response similar to that from a NHNR for a 1 kHz
tone, we could apply a continuously repeating set of four .mu.s
biphasic pulses, where the amplitudes were in the ratio of
4,6.5,7,0, resulting in a neural response in the ratio of
10,24,10,0. This is illustrated in FIGS. 8 and 9.
The stimulation strategy proposed according to the illustrative
example below is designed to be capable of implementation on a
speech processor for cochlear implants which codes signals in terms
of biphasic pulses. The examples illustrated utilise a fixed-width
biphasic--bi-polar pulse, with an overall pulse width of 250
us.
FIG. 4 illustrates a population histogram for a population of 64
nerves around the 1 KHz place in a cat cochlea of total length 2.5
cm. The acoustic input is a 1 KHz tone. FIG. 5 illustrates an
approximation using multiple pulses according to the present
invention.
FIG. 6 illustrates output pulses using one pulse per period, in
other words, using standard stimulation techniques. It is clear
that such fixed-rate stimulation techniques can not form a close
approximation to the desired output histogram at any frequency
other than that of stimulation.
A multiple-pulse electrical stimulation model was iterated in a
trial and error fashion until a set of current levels was found
which provided the required histogram according to the present
invention. The results can be seen in FIG. 7. It is clear that for
each period, the actual histogram closely resembles the desired
approximation of FIG. 5. Clearly, when compared to FIG. 6, the
present invention provides a much closer approximation.
EXAMPLE
The following describes the implementation of the inventive
techniques in relation to a specific sound input.
The phoneme /e (sounds like a short `eh`) is shown in FIG.
13,together with its power spectral density. Note that the spectrum
has a number of peaks in the frequency spectrum (at about 800, 500,
and 200 Hz). These may be used as the main frequencies targeted for
stimulation. The signal is from an isolated sound.
It was applied to the model from Benjamin D Brayant and John D
Gowdy, "Stimulation of Stages I and 11 of Seneff's Auditory Model
(SAM) Using Matlab", published in the proceedings of the 1993
Matlab User's Group Conference.
The model provides the averaged neural response for neurons from
forty regions of the basilar membrane (ranging in characteristic
frequency from high frequency to low frequency). Of course, the
model could be set for any number of bands required, for example,
the response of bands corresponding to each stimulating electrode.
The response of some of the bands are shown as FIG. 14. It will be
appreciated that other models and software could be used to produce
this result.
The inventive technique may be used to code each band which
corresponds to the characteristic frequency of neurons close to an
electrode. For now, let us imagine that band 40 (for instance)
corresponded to an electrode, and examine how the inventive
technique could be used to generate pulses for that electrode. It
will be appreciated that other electrodes would also be coded at
the same time.
In the neural response here, there are two aspects. A broad
lowering of probability over time, probably due to onset effects,
as well as a fine structure. The present invention provides
information to the user about both.
The fine structure of response here has approximately 20 periods in
the 80 ms, corresponding to a period of 4 ms, or a frequency of 250
hz. As an example, we will code with 8 pulses per period, requiring
a coding frequency of 2000 Hz, or in other words a bin size of 0.5
ms.
The probabilities shown in the graph will be the I.sub.k, or NHNR,
of the theory above. So using the formulas given, it is possible to
work out the s.sub.k which will give the required responses.
Let us assume that the population of neurons we wish to control can
be approximated by 3 approximately equally stimulateable regions
(the centre one being the most stimulateable), and we want the
total number of action potentials from this summed population
(divided by the total number of neurons in the summed population)
to follow the curve of the Figure.
Use the following equation, ##EQU5## where S.sub.k,i is the
stimulation function for the ith regionduring pulse k, I.sub.k,i is
the averaging probability of neural response for the ith region
during pulse k, and .gamma.(k) equals one minus the refractory
function evaluated for the case where the last action potential
occurred k.T ago
The probability of firing in each 0.5 ms bin, can then be
calculated, as shown in FIG. 15.
From this calculation, the probabilities required for the first 10
bins are:
______________________________________ k l.sub.k
______________________________________ 1 .014 2 .000 3 .000 4 .022
5 .371 6 .382 7 .231 8 .001 9 .000 10 .000
______________________________________
A given electrical pulse will elicit different responses at
different distances from the site of stimulation. .alpha..sub.i is
defined as the ration between a nominal S for some pulse, and the
actual S generated for the ith region. Let us assume that the
.alpha..sub.i for the three sub-populations are 0.7, 1 and 0.7 for
each population (1, 2, 3).
FIG. 16 gives approximate values of the refractory
function(1-.gamma.), and the gammas would be (very approximately),
about 1 for the first three bins (1 ms), and decreasing from there
to about zero after about 25 ms (the 50th) bin. Therefore,
responses from the last 50 bins are relevant when calculating the
response in any bin.
So gamma will be approximately 1 for the first two bins, and 0.5
after bin 10 or so, and then 0.97 after bin 45, etc.
To obtain the required population per-stimulus time histogram from
the total of the 3 sub-regions, we apply the formula: ##EQU6##
So, to do the calculation:
1. Assume that before time 0 there has been no significant amount
of firing. (If a previous token was coded earlier, then the
processor will remember the I.sub.k 's from that token, and how
long ago).
2. Loop over each pulse, or `k`.
3. Calculate G.sub.k,i. as described above.
4. Using the equation above, calculate the required S.sub.k. Also,
store the three I.sub.k,i values this will evoke for use in
calculations of future G.sub.k,i values.
5. Using the lookup table (as previously discussed) determine the
required intensity of the pulse.
6. Administer the pulse for the correct amount of time, and go to
step 2. This procedure is readily implementable using conventional
software techniques.
Variations and alternatives are possible within the general scope
of this invention, as will be apparent to the reader. In
particular, it is noted that the various processing components may
be differently arranged, so that for example some or all the look
up tables are located within the implanted portion of the
device.
* * * * *